Portable telecommunications devices such as mobile telephones are incorporating an increasing number of functions and applications, such as MP3 players, WCDMA video calling, etc. which are leading to increased competition for the power resources provided by the battery. Many of the loads require large currents, with low voltage drops, for short periods.
Energy management in portable devices usually requires the use of various forms of voltage and current controllers, to stabilise, reduce or increase the voltage provided from the battery, or other energy source, or to control current delivered to a particular load. The voltage and current regulators may be any of, or a mix of, several types, including linear pass regulators, inductive boost, buck or buck/boost converters or capacitive boost, buck or buck/boost converters. These controlling elements have various operational characteristics that must be balanced against needs for efficiency, power delivery, ability and thermal considerations, as well as size and cost.
However, the battery itself, its protection circuitry, battery context and the power distribution circuitry all add resistance to the current path so that, in a typical, small portable device, safety concerns limit the amount of current that can be drawn from the battery to a maximum of approximately two to three amperes. Thus, simultaneous operation of loads which require higher peak power can become difficult, or impossible to manage within the constraints imposed on the battery by size, weight and cost.
Electro-chemical capacitors, often called “super-capacitors”, provide a storage element for electrical energy with an energy density in between that of a battery and a capacitor, and are capable of delivering large currents with low equivalent series resistance (ESR). These super-capacitors can provide larger peak currents with lower voltage reduction than typical lithium-ion secondary cells. Super-capacitors may have a lower voltage rating than the output voltage desired in a system, and may be placed in a series configuration to provide a voltage at multiples of the voltage rating of an individual super-capacitor. Thus, super-capacitors can provide the peak current required, for relatively short periods, and can be charged from an energy source at the average current required by the loads connected to the super-capacitor.
Configurations using super-capacitors use a voltage boosting circuit element to provide a voltage increase to the super-capacitor, and a separate charge balancing circuit ensures the proper distribution of voltage across the capacitors over time. FIG. 1 shows an example of such a configuration. A boost converter 1 boosts a voltage from a battery 2. Two series-connected super-capacitors 3a, 3b are charged by the increased voltage, and power can be drawn from either super capacitor 3a, 3b to provide a high current for a short period to a load. A charge balancing circuit 4, in this case comprising a voltage divider and an opamp driver, balances the charge between the two super-capacitors 3a, 3b. 